A centering device is arranged between the rotary slide and the valve output element in support of the torsion rod spring. This device contains two centering elements, which are rotatable against each other, and there is at least one anti-friction element located between the two centering elements. One of the centering elements forms a rotationfixed link with a reaction piston.
Such a rotary shift or slide valve is known from DE-A1-44 33 599. The advantage of this rotary shift valve is that the outer dimensions of the entire valve can be very small, which is due to the particular arrangement of the control bush inside the rotary slide. With this rotary shift valve, centering is effected via the torsion rod spring. The particular feature of a torsion rod spring is that the return effected by the torsion rod spring in the actual center area of the rotary shift valve is only minimal. Therefore, the torsion rod spring is supplemented by a centering device, which allows precise centering, especially in the center area of the rotary shift valve. In this version, extra space in axial direction is required for the additional centering device, and especially for longitudinal guidance of one of the centering elements.
Since the centering device is arranged between the rotary slide and the valve output element, it is possible for a prefixed position to be determined for one centering element of the centering device together with the valve output element. As a result, it is possible to set centering force and position of the centering device after production of the individual parts. Since the reaction piston rotates together with the rotary slide, a relative motion results between the reaction piston and a valve housing. The reaction piston must be sealed off against the valve housing, and therefore friction arises, which directly affects the manual force to be applied to a hand wheel.